Cloned DNA fragments from Herpes Simplex Type I virus, originating from various regions of the viral chromosome will be tested for their ability to replicate autonomously in mammalian cells. Naturally arising 'defective' HSU-1 chromsomes will be similarly tested. Enrichment for cells containing these fragments will be based on the observation by others that thymidine-kinase positive transformants of TK-cell lines have often picked up other exogenous DNA fragments. Assay for such fragments in TK ion transformants will be by standard gel electrophoretic and bouyant density banding techniques.